1. Field of the Invention
The present invention relates to the field of wireless communications and, more particularly to a single-source frequency diverse power control method and apparatus for CDMA wireless cellular handsets.
2. Description of the Related Art
Code Division Multiple Access (CDMA) transmission schemes have become increasingly popular due to the recent growth of the cellular industry. CDMA is a spread spectrum technique whereby data signals are modulated by a pseudo-random signal, known as a spreading code, before transmission. The modulation of the data signals spreads the spectrum of the signals and makes them appear like noise to an ordinary receiver. When the same pseudo-random signal is used to demodulate (despread) the transmitted data signal at the CDMA receiver, the data signal can be easily recovered.
Currently, there exists an industry standard requiring CDMA handset transmitters to achieve a minimum of 73 dB of output power control. As is known in the art, the terms xe2x80x9cgain controlxe2x80x9d and xe2x80x9cpower controlxe2x80x9d may be used synonymously since gain can be translated into power at any point during the transmit chain. The maximum required gain minus the minimum required gain is often referred to as gain range. In practice, the circuit components employed to achieve this output power control must also achieve additional power control to compensate for device, frequency, mode and temperature variations. These variations increase the minimum output power control of the components used in the CDMA transmitter to slightly over 100 dB of gain range.
FIG. 1 illustrates a typical transmit chain 100 used in a CDMA handset transmitter. The transmit chain 100 includes a digital-to-analog converter (DAC) 102, modulator 104, an intermediate frequency (IF) stage 116 and a radio frequency (RF) stage 118. The IF stage 116 includes an IF amplifier 106 and an IF filter 108. The RF stage 118 includes a RF upconverter 110, oscillator 112 and RF amplifier 114.
The DAC 102 receives digital baseband data from a remaining portion of the CDMA handset. Typically, the baseband data is received from a microcontroller or processor, such as a digital signal processor, responsible for controlling the operation of the handset. The baseband data is comprised of two signals known in the art as the in-phase I and quadrature Q signals. The DAC 102 converts the digital baseband data into analog I and Q signals and outputs the analog I and Q signals to the modulator 104.
The modulator 104 inputs the analog I and Q signals, combines and modulates the signals into one IF signal which output to the IF amplifier 106. The IF amplifier 106 has a gain controlled by a control signal. The IF amplifier 106 amplifies the IF signal and outputs it to the IF filter 108. The IF filter 108 filters out any noise from the amplified IF signal and outputs the filtered IF signal to the RF upconverter 110. As known in the art, the upconverter 110 converts the amplified IF signal into a RF signal. This conversion is controlled in part by the oscillator 112 connected to the upconverter 110. The RF signal is output by the upconverter 110 to the RF amplifier 114. The RF amplifier 114 has a gain controlled by a control signal. The RF amplifier 114 amplifies the RF signal such that the transmission power of the RF signal has the desired output power. This signal would then be supplied to an antenna where it is radiated to a CDMA base station.
Allocation of both gain and gain range through the transmit chain 100 results from tradeoffs based on the noise performance, linearity, current consumption and isolation issues of the chain 100. Noise performance and linearity are known to have the biggest impact on system performance.
A CDMA system or handset is a full duplex system, that is, both the transmitter and receiver are operating simultaneously. In a CDMA handset, the transmit chain 100 must be designed to eliminate noise appearing at the receiver frequency band. This noise would interfere with a RF received signal. Therefore, the design of the transmitter must be such that its thermal noise is much lower than the thermal noise generated in the receiver. It is this constraint which drives the noise figure requirements, the IF gain allocation and is a critical factor in determining the electrical characteristics of the IF filter 108. The high gain range is the constraint which forces the output power control to be performed across the two stages 116 (i.e., a two-stage power or gain control) , 118 as opposed to being performed in either the RF or IF stage (i.e., a one stage power or gain control).
Today, almost all power control is performed via a two-stage power control having a variable gain. Typically, these methods utilize separate signals to control the gain of the two stages. It is desirable, however, to control the two frequency-diverse variable-gain stages (i.e., the IF and RF stages) with a single control signal. The use of one control signal would greatly enhance the overall handset design and circuitry by simplifying the interface between the transmit chain and the handset micro-controller. This would improve the cost associated with manufacturing the handset as well as its performance.
FIG. 2 is a block diagram illustrating an exemplary automatic and adjustable power control (APC) circuit 120 for controlling the two frequency-diverse variable-gain stages (i.e., the IF and RF stages 116, 118) of the transmit chain with a single APC control signal VapcMaster. The APC circuit 120 is implemented either in analog or digital circuitry. As shown in FIG. 2, the APC circuit 120 utilizes the VapcMaster signal to generate a first signal VapcIF to control the gain of the IF amplifier 106 and a second signal VapcRF to control the gain of the RF amplifier 114. The VapcMaster signal is an analog voltage level whose amplitude is output by the handset micro-controller. As described below, this signal is used by the APC circuit 120 to generate the VapcIF and VapcRF control signals which are then respectively applied to the IF and RF amplifiers 106, 114. In most wireless handset applications, the VapcMaster signal will be approximately 2.0 volts at maximum.
One method of controlling the gains of the IF and RF stages 116, 118 is by a sequential control method. This sequential control method varies the gain of one of the stages over the stages entire gain range prior to xe2x80x9chanding offxe2x80x9d the power control to the other stage. For example, the method would begin by varying the gain of the RF stage over the entire RF stage gain range. When this is complete, the method would continue by varying the gain of the IF stage over the entire IF stage gain range. During this method, the total gain GTot, which is the addition of the gains of the IF and RF stages, must remain within the required gain range. The gain control of the sequential method is performed by the APC circuit as follows:
GTot=GIF+GRF=VapcIF*GSIF+VapcRF*GSRF, where
VapcIF=VapcMaster, VapcRF=VapcRFxe2x88x92min when VapcMasterMin less than VapcMaster less than APC Handoff Level,
VapcIF=VapcIFxe2x88x92max, VapcRF=VapcMaster when APC Handoff Level less than VapcMaster less than VapcMasterMax,
GSIF=the gain slope of the IF stage in dB/V=(total gain range of IF stage)/(control range of the IF stage in Volts) and
GSRF=the gain slope of the RF stage in dB/V=(total gain range of RF stage)/(control range of the RF stage in Volts).
It must be noted that the APC Handoff Level is a voltage level of the APC control signal VapcMaster at which the RF gain range has been completely exercised. Once the VapcMaster reaches the APC Handoff Level, the sequential method begins to exercise the gain of the IF stage over its entire gain range.
A second method of controlling the gains of the IF and RF stages 116, 118 is by a simultaneous control method. In the simultaneous control method the control lines for the RF and IF stages 118, 116 are both tied to the master control signal VapcMaster. Both stages 116, 118 would operate independently of each other since each stage""s control voltage VapcIF, VapcRF equals the master control signal VapcMaster. During this method, the total gain GTot, which is the addition of the gains of the IF and RF stages, must remain within the required gain range. The gain control of the simultaneous method is performed by the APC as follows:
GTot=GIF+GRF=VapcIF*GSIF+VapcRF*GSRF, where
VapcIF=VapcRF=VapcMaster,
GSIF=the gain slope of the IF stage in dB/V=(total gain range of IF stage)/(control range of the IF stage in Volts),
GSRF=the gain slope of the RF stage in dB/V=(total gain range of RF stage)/(control range of the RF stage in Volts) and
GSIF=GSRF is not generally true.
It must be noted that the simultaneous method does not hand-off control as performed by the sequential method. Accordingly, there is no APC Handoff Level for VapcMaster in the simultaneous method.
A third possible gain control method would utilize a combination of the sequential and simultaneous methods. This would be difficult to implement, however, since simultaneous control requires VapcIF=VapcRF while the sequential methods requires the two control voltages VapcIF and VapcRF to be independent of each other.
These methods, however, have some shortcomings which will become evident after a brief description of four major transmit chain design constraints. The first constraint (hereinafter referred to as xe2x80x9cconstraint #1xe2x80x9d) mandates that as the gain is lowered, the current consumption must also be lowered. This allows for battery conservation and is easily implemented as part of the APC control scheme. For power conservation reasons, the stage which consumes more power ideally has its gain lowered first so that the circuitry reduces its current consumption as quickly as possible when the total gain is reduced from its maximum. This will be the RF stage in most transmit chains architectures.
The second constraint (hereinafter referred to as xe2x80x9cconstraint #2xe2x80x9d) mandates that, for noise reasons, it is desirable to lower the gain of the RF stage since it makes a greater contribution to the output noise floor.
The third constraint (hereinafter referred to as xe2x80x9cconstraint #3xe2x80x9d) mandates that, for linearity reasons, it is desirable to lower the gain of the IF stage first since its output feeds the RF stage in a cascaded manner. A lowering of the IF input to the RF stage without lowering the operating point of the RF stage allows the RF stage to operate in a more linear fashion, since the output power level is farther away from the non-linear range.
The fourth constraint (hereinafter referred to as xe2x80x9cconstraint #4xe2x80x9d) mandates that, to compensate for variations in power levels along the transmit chain, it is desirable to extend both the IF and RF gain ranges. The extension of the IF gain range beyond what is required by the system specification indicates that under some conditions the IF output power will overdrive the RF stage causing non-linear operation and non-compliant adjacent channel emissions.
When compared with these designs constraints, it is evident that an RF-first sequential method would satisfy constraints #1 and #2 only while an IF-first sequential method would satisfy constraints #3 and #4 only. The simultaneous method would compromise all four of the constraints without properly satisfying any of them. The compromise method would be difficult to implement and would not optimize system performance with respect to the four design constraints. Accordingly, there is a desire and need for a power control scheme for a CDMA handset that utilizes a single control signal and provides optimal output power control.
In view of the foregoing shortcomings, and for other reasons, the present invention is directed to a CDMA cellular handset with a simplified design for optimally controlling the output power of the handset. The invention comprises an automatic and adjustable power control method and apparatus that utilizes a single control signal to control two frequency diverse variable gain stages of a transmitting portion of the handset by a multi-point hand-off technique.
In one aspect of the present invention, an automatic and adjustable output power control method for a telephone handset is provided. The method includes the steps of providing a control signal; altering a gain of a first transmitting stage of the handset to satisfy at least one system parameter in response to a first variation of the control signal; altering a gain of a second transmitting stage of the handset to satisfy at least one additional system parameter in response to a second variation of the control signal; and varying the altered gain of the first transmitting stage in response to a third variation of the control signal.
In another aspect of the present invention, an apparatus for providing automatic and adjustable output power control to a transmitting portion of a telephone handset is provided. The apparatus includes a controller coupled to the transmitting portion of the handset. The controller receives a control signal from a second portion of the handset and: altering a gain of a first transmitting stage of the transmitting portion to satisfy at least one system parameter in response to a first variation of said control signal; altering a gain of a second transmitting stage of the transmitting portion to satisfy at least one additional system parameter in response to a second variation of said control signal; and varying said altered gain of the first transmitting stage in response to a third variation of said control signal.
In yet another aspect of the present invention, a telephone handset is provided. The handset includes a first controller providing a control signal; a transmitting circuit, said transmitting circuit including first and second transmitting stages, said transmitting circuit having an output power; and a power control circuit coupled to said transmitting circuit and said first controller. The power control circuit includes a second controller coupled to said first and second transmitting stages receiving said control signal from said first controller, said controller: altering a gain of said first transmitting stage to satisfy at least one system parameter in response to a first variation of said control signal; altering a gain of said second transmitting stage to satisfy at least one additional system parameter in response to a second variation of said control signal; and varying said altered gain of said first transmitting stage in response to a third variation of said control signal.
It is an object of the present invention is to provide an apparatus for controlling the output power of a wireless handset in an optimal manner.
It is another object of the present invention to provide an apparatus for controlling the output power of a wireless handset that simplifies the overall design of the handset.
It is yet another object of the present invention to provide an apparatus for controlling the output power of a wireless handset that reduces the overall cost of the handset.
It is a further object of the present invention is to provide a method for controlling the output power of a wireless handset in an optimal manner.